1. Field of the Invention
The present invention relates to detection of sounds and augmentation of detected sounds above ambient noise. In a class of embodiments, it relates to detection of sounds propagating from within human or animal bodies (e.g., sounds from the heart or lungs) using an active stethoscope configured to augment the sounds of interest above ambient noise.
2. Prior Art
Throughout this disclosure, including in the claims, the expression “active” stethoscope (or “active” sound detection device) denotes a stethoscope (or sound detection device) that includes an acoustic transducer useful for converting acoustic waves (e.g., body sounds of interest) into another form of energy.
Herein, the expression “electronic” stethoscope (or “electronic” sound detection device) denotes a stethoscope (or sound detection device) that includes an acoustic transducer useful for converting acoustic waves of interest (e.g., body sounds) into at least one electric signal. Also herein, the expression “passive” stethoscope (or “passive” sound detection device) denotes a stethoscope (or sound detection device) that does not include an acoustic transducer.
Throughout this disclosure, including in the claims, each of the expressions “acoustic transducer” and “sound transducer” denotes a device for converting acoustic waves into another form of energy. For example, one type of acoustic transducer is a typical microphone configured to convert acoustic waves into an electrical signal. Another example of an acoustic transducer is a device configured to convert acoustic waves into electromagnetic waves (e.g., visible radiation or electromagnetic radiation whose wavelength or wavelengths is or are outside the visible range), and optionally also to convert the electromagnetic waves into an electrical signal.
Acoustic transducers are sometimes referred to herein as sound pick-ups, and are sometime referred to herein simply as transducers.
Throughout this disclosure including in the claims, the expression that a first element is “mounted to” a second element denotes that the first element is attached or coupled in any manner to the second element at at least one point or region of the second element (each such point or region is denoted herein as a “coupling point”), such that when the second element moves (e.g., vibrates), the first element moves in phase with and in sympathy with the second element at each coupling point. A first element can be “mounted to” a second element if the two elements are directly attached to each other or if they are otherwise coupled to each other (e.g., coupled by any rigid coupling means) without freedom to move relative to each other at each coupling point. A first element can be “mounted to” a diaphragm (a flexible element) at at least one coupling point even if portions of the diaphragm other than each coupling point have freedom to move out of phase with the first element.
The expressions “mounted on” and “referenced to” are used herein as synonyms to the expression “mounted to,” with reference to a floating mass that is mounted to a diaphragm.
Active and passive stethoscopes are used by health care givers (to be referred to herein as physicians since they are typically physicians) to aid in the detection of body sounds for the purpose of diagnosing various symptoms, for example, heart beat anomalies or lung infections. This procedure is commonly called auscultation. Stethoscope design is a specialty art, difficult to learn due to the low sound levels emitted by the body. Electronic stethoscopes have been used in the medical field for some time, with mixed success because body sound emissions are typically only a couple of Decibels (dB) above the background noise. Background noise is more precisely known as noise floor and will be referred as such in this disclosure.
The conventional stethoscope in its most primitive form consists of a closed space (often referred to as a “chest piece”), generally shaped like a round pill box, with one side consisting of a semi-flexible diaphragm and another side having a flexible tube attached. The flexible tube is generally molded at the far end into a “Y” shape which is applied to the physician's ears. In use, the diaphragm side of the chest piece is placed against the body surface so that sounds from the body cause the diaphragm to move in sympathy. The air space in the interior of the chest piece experiences minute pressure waves from the moving diaphragm. These pressure waves travel up the flexible tube and are perceived as sound by the physician's ears.
Such a conventional, passive stethoscope has several drawbacks, including the following: the perceived sound is very, very low in amplitude; the sound is colored by the absorptive characteristic of the flexible tube; and the sound is colored by the resonant characteristics of the air column in the flexible tube. Attempts have been made to improve these drawbacks such as careful material selection and finish of said flexible tube.
An additional and major improvement was to size the flexible tube of a passive stethoscope so its air column resonated at about 50 to 100 Hertz. (One company among many that specializes in this area is 3M Corporation with a line of stethoscopes generally called the Littmann line). The resonant air column augmented the detection of the heart beat greatly. However, the entire stethoscope's frequency response was colored towards the 50 to 100 Hertz frequency and its multiples. A recent example of a passive stethoscope having a resonant cavity for emphasizing detected sound frequencies within at least one predetermined frequency range is described in U.S. Pat. No. 4,270,627, issued to Raymond R. Hill. The diaphragm of the stethoscope described in the Hill patent (and other conventional stethoscopes) can include a thin, distally protruding probe attached to its center. In use, the probe moves in sympathy with the body sounds being detected, and thus the probe cannot have a high mass.
Much if not all of the medical community has been trained (e.g., many if not all cardiologists have been trained) using stethoscopes designed to emphasize detected sound frequencies within a predetermined frequency range (typically from about 50 to 100 Hertz) and in fact, a pure and perfect response stethoscope would sound strange to them. This pitfall was experienced by the earliest electronic stethoscopes.
An example of an electronic stethoscope designed with a goal of achieving purity in auscultation, without emphasizing detected sounds in a predetermined frequency range, is described in U.S. Pat. No. 6,498,854, issued Dec. 24, 2002, to Clive Smith. This patent teaches in detail the purity aspects of a totally unloaded stethoscope diaphragm. The diaphragm functions as an electrode of a capacitor, and its movement in response to body sounds is converted to an electric signal.
In practice it has been found favorable to design an electronic stethoscope which mimics the response of Littmann brand passive stethoscopes. An electronic stethoscope that mimics a resonant tube passive stethoscope is described in U.S. Pat. No. 6,587,564, issued Jul. 1, 2003, to Ronald Cusson. This patent describes a resonant chamber sound pick-up for an electronic stethoscope, including metal ballast, a sound pick-up rigidly attached to the ballast, a support cup, and closed cell, compliant foam between the ballast and support cup. A diaphragm (whose distal surface is designed to be placed against the patient's skin during use) is mounted to the compliant foam so as to define a resonant cavity between the sound pick-up and the diaphragm's proximal surface.
FIG. 1 is a stylized cross-sectional view of a first prior art electronic stethoscope chest piece which comprises a sound transducer 1 (which is typically a miniature microphone) mounted in a rigid body 3 held in a chest piece housing 4. Diaphragm 2 is mounted in front of (distally with respect to) transducer 1. Diaphragm 2 is held to housing 4 by a generally circular ring 9. Rigid body 3 is fastened to housing 4 by bosses 8 or some other rigid means.
The electronic stethoscope chest piece of FIG. 1 also includes electronic amplifier 5 and power source 6 for amplifier 5 (the power source is usually a battery). Amplified signals from microphone (or other transducer) 1 are sent along wires from amplifier 5 through stethoscope tube 7 to sound transducers (not shown), which are generally headphones, located at a stethoscope head piece (not shown) at the proximal end of stethoscope tube 7.
The entire construction of the FIG. 1 chest piece is relatively rigid. Diaphragm 2 is generally rigidly fastened to housing 4. Microphone (or other transducer) 1 has a largely isotropic characteristic. It picks up sound vibrations from all directions which include the diaphragm 2, rigid body 3, and housing 4. This first prior art electronic stethoscope chest piece has the poorest signal to noise performance of those described herein.
FIG. 2 is a stylized cross-sectional view of a second prior art electronic stethoscope chest piece, of the type described in above-cited U.S. Pat. No. 6,587,564 to Cusson. The chest piece of FIG. 2 comprises sound transducer 11 (typically a miniature microphone) mounted in a rigid body 13 held in a closed cell, compliant foam cup 18. This assembly is held in housing 14. In front of microphone 11 is placed a diaphragm 12 which is directly attached to foam cup 18, but not to rigid body 13. In this second prior art example, diaphragm 12 is not rigidly fastened to housing 14. The chest piece of FIG. 2 also includes electronic amplifier 15 and power source 16 for amplifier 15 (the power source is typically a battery). Amplified signals from microphone (or other transducer) 11 are sent along wires from amplifier 15 through stethoscope tube 17 to sound transducers (not shown), which are generally headphones, located at a stethoscope head piece (not shown) at the proximal end of stethoscope tube 17.
The prior art chest piece of FIG. 2 has significantly better signal to noise performance than the chest piece of FIG. 1 for two reasons: first, rigid body 13 of FIG. 2 is not rigidly fastened to housing 14 (whereas rigid body 3 of FIG. 1 is rigidly attached to housing 4); and second, diaphragm 12 of FIG. 2 is not rigidly fastened to housing 14 (whereas diaphragm 2 of FIG. 1 is rigidly attached to housing 4). The prior art chest piece of FIG. 2 also utilizes a resonant sound chamber 19 (and has a narrow vent that extends horizontally from the proximal wall of chamber 19) to augment frequencies in the 50 to 150 Hz range. An electronic stethoscope including the chest piece of FIG. 2 typically performs significantly better than one including the prior art chest piece of FIG. 1.
Most electronic stethoscopes exhibit poor signal-to-noise performance. The sound of the human heart and other body sounds at the skin level are only a dB or two above the threshold of human hearing. Also, general ambient noise is present at this low level. This presents a difficult, two-fold problem that affects all stethoscopes: (a) how to hear the low level (and typically very faint) heart sound or other body sound of interest; and (b) how to discriminate the body sound of interest from ambient noise. Other conventional sound detection devices for detecting low level sounds in the presence of noise are subject to this two-fold problem. The present invention addresses the problem in several ways.